We have decided that once their bags are nearly finished we will be moving our boys on to Wafcol. Our eldest is 11 months and very lean and our youngest is 3 months old. Would it be possible to feed them both the same variety?

Your youngest at 3 months definitely needs the puppy variety.  I know it depends on breed at what age a dog moves onto adult variety, but if your 11 month boy won't hurt for bulking out, I see no reason at all why he can't go on the same until he's all growed up.

It definately wouldn't hurt him to gain a bit, to be honest he has never been a great eater (the same can't be said for our new baby! lol)