"thoughts on a lightweight 17' tandem that can be used for a solos/tandem trips?"

guessing at your intended use, to be mostly a smaller tandem for your kids as they transition and  for solo ?

only two boats I can think of that would work for that

Bell's Northstar Canoe - Northwind 16 - 16'6"  it is likely identical to the old Bell Canoe Works Northstar model at 16'6 which I have in Kevlar Ultralight.  recommended for smaller paddlers and light loads, BUT It is the boat I use for trips with my brother for 2 weeks or so - it is quick, as it is narrower than most 17 footers.  It can be bought with optional center seat or kneeling thwart.  I don't have either, but have soloed it a couple of times (only day use) from the thwart that would be the kneeling thwart if i had one - yeah, you can solo it, but any solo canoe would do better.  works best as a tandem.

about the same can be said for Wenonahs Solo Plus, though I have never paddled one. 

Souris River Q 16 comes in both a tandem and solo version - I'd guess that might be a contender as well if you set it up for both tandem and solo.

problem with three seats in a boat is it takes away storage space, and probably makes for more difficult portaging.  If I were to have a boat like that, I'd probably rig a backpack frame carrier that hooked to the center seat for portaging.

"Center" seat, should more likely be behind center a bit, not right at center for soloing.

Nanda - you Really Really don't want to solo an 18 footer or even 17 footer tandem - maybe if you are just going out to a farm pond, but otherwise not - way too much surface area for the wind to blow you around - it also takes more horsepower to get the bigger boats up to cruising speed

the Wenona Prism and Wilderness are both solo canoes - can't be used for tandem for any practical purposes

I've once used a solo boat for tandem on a local lake - you have to sit on the floor and it is harder to paddle that way - also decreases stability