Like I said, Sony can manage it, no reason MS can't. Some of the multiplatform games have been part of PS+ like Contrast, Outlast etc.
They should be investing in indies like Sony did to bolster their early offers until they can include retail X1 games, avoiding this very situation. Basically what we're seeing is a premature offer / programme instead.
And if they couldn't, there's no reason they can't offer more 360 retail / XBLA titles to make up for it. It's not about wanting more more more, but at least match them when you're playing catch up and have the advantage.
They need to have something for selling when they launch XBL in remaining markets in next few months. Sony doesn't have that factor because they have launched pretty much everywhere they usualy do (except China?).
Their retail offerings are as good as Sony if not better but if they give away everything now, what will they sell come Aug-Sept?