Our twins are just over eighteen months and are having issues sharing. Given their shared age the answers provided to "how do you encourage siblings to share?" is not specifically aplicable. So far it's mostly minor and we've been able to coax the children into doing the "right" thing when they've taken something from the other. However, we see this sharing issue as having the potential for becoming a larger problem. How can we encourage sharing between toddler twins?
Developmentally they're at the age where sharing is an alien thing, so what you're seeing isn't necessarily a bellweather for problems down the road.
That being said, the best luck we've had teaching our 21-month-old to share is modelling the desired behavior. When he picks up something that's interesting to him but that we'd rather he didn't play with (the remote, for example) we ask him if we can have it. He brings it over, we take it and thank him PROFUSELY for sharing. Same for his sister; if he picks up her lovey, he brings it to her and gets HUGE thanks for sharing. It's definitely sinking in; all I can hope is it sticks when he hits the "mine" phase.
I have twins who are 2.5 years old. At 18 months consider having the kids take turns with a toy and not play with it at the same time (another form of sharing). First it's Sam's turn and then it's Alex's. You can use a timer so after 2 minutes they have to switch toys. Be very encouraging, thanking them for sharing and playing nicely together. At this age trading is better than sharing. If Sam has something Alex wants encourage Alex to offer him a toy in exchange for a turn. Finally, use the word sharing in your own life. If another adult passes you the salt during dinner thank him/her for sharing.
I agree with Valkyrie.
To add a technique, tho, you could also try sitting and playing with one or the both of them a few times. Emphasize when you're giving stuff to them. Emphasize when you don't want them taking the knives that you're playing with at that particular moment, especially when there's other knives on the floor between you just itching to be played with.
Or... maybe not knives. Bandsaws.