You may very well find that 1.3 compared to a 3 watt are not well matched if you're into things being symmetrical. So yes, if you buy a 3 watt, you're probably setting yourself up. You may even find a 2 watt to be out of balance. If I had the opportunity to do it over, I would have bought two identical projectors at the same time. As an example, I have two Kvant Spectrum 1.6's and even they are not a match since they used different combinations to acheive 1.6. For me... it's frustrating. For an audience, they probably don't notice or care.
Personally, I would daisy chain rather than use a Y splitter at the controller. In all likelyhood, you need less ILDA cable that way and less chance of people tripping over cable. As far as the mirroring - since you're inquiring about a new projector, ask the manufacturer if they have X Y invert switches or, if they could add them. You might also inquire about external pots for adjusting your RGB levels. They probably don't do that but it doesn't hurt to ask. You could go the pin reversed wire route or, if you're handy put your own invert switches into one of the projectors.
Since you've only stated "hong kong", I'll also suggest you look at
www.goldenstarlaser.com before purchasing anything.