According to both FINAL FANTASY XIV: A REALM REBORN OFFICIAL COMPLETE GUIDE [March 2015, Japanese Lang.] and FINAL FANTASY XIV: HEAVENSWARD CRAFTER & GATHERER & GOLDSAUCER OFFICIAL GUIDE [March 2016. Japanese Lang.] that intercross of a plant is determined in a fixed specific order (see below). Hence, the intercrossing path is PREDETERMINED and not randomly selected as previously thought. This also explains why previously intercrosses were thought to be One-way and not Two-Ways.
Find following a capture from the 2016 Guide Gardening section.
With Eliza's help, from the Japanese community of FFXIV players. The following was translated:
"In the case where seed 'C' is planted in bed (2) between plant 'A' and plant 'B', then 'C' will prefer to intercross with plant 'B', unless there is no chance to intercross with 'B' then it will move to cross with 'A'.
The manner that the plant chooses which course to intercross with is as follows: Right > Down > Up > Left
Figure is a hypothetical garden patch; Planted seed in centre will seek adjacent plant to intercross with, starting with 1 (right), if the intercross is null, it will move to 2 (down), 3 (up), & 4 (left). It will stop where it will have a chance to intercross. Note: percentage of success is determined based on Topsoil of choice.
You will also need to make sure you orient your garden patch accordingly so you know where Bed #1 is located. You can check this by selecting a bed. The dialog window will show you which bed and patch you are at (eg. 1st Bed, 2nd Patch).
Special thanks to Eliza Weisenberg (Aegis / Elemental DC) for translation and captures.
Consider the following scenario of an Oblong Garden Patch (6 Bed) where a [La Noscean Orange] plant, a [Pixie Plum] plant, and a [Honey Lemon Plant] exist as following (figure 1). The remaining beds are left empty for simplicity purpose.
In case an [Old World Fig] seed is planted in Bed #2, as follows (figure 2):
Applying Priority Path (Right, Down, Up, Left); the [Old World Fig] plant will attempt to intercross with the [Honey Lemon] since it is on the right and will have a chance to yield an [Umbrella Fig] seed*. There is no chance that the [Old World Fig] plant will intercross with any of [Pixie Plum] nor [La Noscean Orange].
In case that the [Honey Lemon] did not exist in bed #3, and an [Old World Fig] seed is planted in Bed #2 (figure 3):
Applying Priority Path (Right, Down, Up, Left); the [Old World Fig] plant will skip the bed on the right and move to the below bed and will attempt to intercross with the [Pixie Plum] and will have a chance to yield a [Royal Kukuru] seed*. Similarly, there is no chance that the [Old World Fig] plant will intercross with the [La Noscean Orange].
In case that also the [Pixie Plum] did not exist in bed #5, and an [Old World Fig] seed is planted in Bed #2 (figure 4):
Applying Priority Path (Right, Down, Up, Left); the [Old World Fig] plant will skip the beds on the right, below it, above it (out of range) and move to the bed on the left and will attempt to intercross with the [La Noscean Orange] and will have a chance to yield a [Apricot] kernel*.
Last case, when an [Old World Fig] seed is planted in Bed #2, between one or more plants that does not have a known crossbreed (dead cross), as follows (figure 5):
Applying Priority Path (Right, Down, Up, Left); the [Old World Fig] plant will skip the bed on the right (dead cross) and treat it as an empty bed, below it (also dead cross) and treat is as an empty bed, above it (out of range) and move to the bed on the left and will attempt to intercross with the [La Noscean Orange] and will have a chance to yield a [Apricot] kernel* as in the previous case.
* Percentage of intercross success is affected by the Type and Grade of Topsoil used.
Since this was only mentioned in the HEAVENSWARD Guide, I went and tested it out to verify that it is still valid in September 2020 & current expansion of SHADOWBRINGERS. In this verification test I will be planting 20 set ups of: [La Noscean Orange] [Old World Fig] [Pixie Plum] as follows:
Where [Old World Fig] is the crossing plant, according to the Path Priority Order it should ALWAYS cross to the RIGHT. Hence yielding [Royal Kukuru] seeds if cross happens. Otherwise if an [Apricot] kernel is observed, that will verify that this Path Priority Order does not apply any longer.
With a 90% chance of intercross (using Grade 3 Thanalan top soil), I should end up -theoretically- with:
Royal Kukuru Seeds x18
0% chance of getting Apricot Seeds
If successful, this will demolish the previous thought equal chances intercross of a plant with any random adjacent bed & affix it to the Priority Path Order to be the true path to intercross.
In Probability terms, acquiring 18 consecutive same result (seed) is 1 out of 260,144 if you assume there is a 50-50 chance that Old World Fig will cross either La Noscean Orange or the Pixie Plum.
ONE Apricot seed will crush the whole theory and official statement in the 2016 guide. Remember that all the game and guide was made by human and its always subjected to Human-Error, as much as subjected to change and updating at any point in time.
Testings Results For a 50-50 chance to get 17 seeds in a row is 1 out of 131,072 chance!
Verifying that the Priority Path Order is still Valid to date.