Layla sees the MC kiss her BFF and suck her titties and it didn't change her mind that the MC is worth taking her cherry, all the jealousy she has towards other girls doesn't extend to Elena.
And they are often kissing each other anyway...
If you said to Layla that you prefer Elena it's tough to come back from that... you basically told her she's always going to be the third wheel if sharing were to happen, not to mention she's still a virgin.
With Layla as the first i think it's easier to convince Elena that you love both equally, you just happened to meet Layla first.
This seems the more believable possibility of sharing, compare this to the suppousedly BFFs Alexis and Erynn and it's a completely different picture, some love but also a little bit of a cold war between those 2 (hopefully i can make nukes start flying
).