I think any 'occasion' is what you want it to be.
I'm not Christian but I celebrate Christmas for what it means to me.
If you always celebrated and enjoyed celebrating Valentine's day before, I suggest you continue. Not everything needs to change because you're poly, if you like it, celebrate it.
Time and sex for both on one day is difficult though, so seeing them on separate days is probably for the best. But this is the same for a lot of relationships, I often have a lot of stuff on/am away during 'actual' valentine's day, so if I do celebrate it, it's on a different day. Although I've never really celebrated it the 'proper' way anyway. First one I celebrated, I got my boyfriend at the time a pair of leather gloves rather than chocolate or a card and last valentine's I sent my boyfriend a joke card with a photo of the two of us the front and a caption that said:
'Roses are red,
Violets are blue,
I like girls,
I guess you'll do.'
He completely cracked and up reading it and it's now pride of place on our mantelpiece :P
Maybe, on the day itself, you could send them both a card/chocolates/anything else you think would make them feel loved an appreciated.