Tandem Cycling - MY trek T900

I bought it two weeks ago, sunrace 27-speed replaced SRAM 24-speed, I alway rode it with my wife on mountain road, tha't great, but the wheelset is so heavy, can I use my lightweight MTB wheelset in T900?
http://nantou.freeforum.tw/up/fftwpw608005/46/74/4674//thumb/08/51/4183_1_70252f21b8c3f0a.jpg

Depends on team weight and how 'light' the light weight wheelset is. In general
it is not likely to make as much difference as you might think in the bike ride.
You need to check the rear triangle width as well, might be MTB width but might
be a bit wider. You could split the difference and use the front MTB light wgt
wheel only. Front wheel is less stressed than the rear on any bike and axle width
is same tandem and singleton.

Spoke and rim failures are a plague in tandems which use single bike wheel sets. I see that here where tandem specific parts are not available and people build up tandems with the available parts.

I bought it two weeks ago, sunrace 27-speed replaced SRAM 24-speed, I alway rode it with my wife on mountain road, tha't great, but the wheelset is so heavy, can I use my lightweight MTB wheelset in T900?

You might just want to change out the tires, as that would probably to more to change the way that bike rides vs. anything else I can think of. Those stock tires are VERY heavy and stiff, which is most likely what you're experiencing.

As for using your other wheels, assuming the T900 uses 135mm rear drop-outs (off-hand, I don't know) you'd need to check to see if there is a max weight limitation on your MTB wheels and then see how that compares to the total weight of your full loaded tandem. If the drop-out spacing doesn't match up within just a few mm's, I'd leave it alone and, again, just revisit the tires.

I am 80KG, my wife is 50KG, I'll put my lightweight wheelset on T900 to try and change tires ....ask T900 owner,s, by the way, is it worth to change suspension fork ?

T900 rear OLD=145 mm. A standard MTB rear wheel (OLD=135 mm) is not compatible without modification.
At your team weight, you can certainly use the lighter front wheel. And, as TandemGeek suggests, lighter tires are readily available. If you ride primarily on the road, consider the Bontrager "Road Warrior" 26x1.25. The sidewall says 45-85 psi. We run 'em at 100 psi on our triple, with a team weight of 330 lbs/150KG.
We do not own a T900, but that won't stop me from offering this opinion: A suspension fork will add weight, complexity, and cost. It will also change geometry and handling. If you do not ride off-road very often, you can surely do without one.

the other question, after I changed it to 27 speed, the Front Derailleur is hard to shift geers, I mean it's hard to shift to small chain ring, someone told me some mistakes
1.maybe the BB's width is too short..but I never change BB, It works good in 24-speed
2.maybe the 9-speed chain is thiner than the 8-speed chain, front deraileur is hard to push chain to the small chain ring
3.maybe the crankset (Truvativ FiveD tandem) only work in 24-speed

what should I do?