Alright....PUMP-->RADIATOR-->PRIMARY GPU-->SECONDARY GPU-->T-LINE-->PUMP. Having the T-line between the gpu blocks would restrict your flow too much. If you insist on tubing it that way, use Y fittings...they don't restrict flow as much. Also, sinve you are using 68's, i would reccommend DangerDen NV-68s,,,they woud rectify the flow problem between the vidcards and they are highly efficient.
EDIT: Use a t-line, not a res. They are cheaper and IMO easier to bleed since they do it on the fly. After you first start your setup, leave your T-line elevated and uncovered for @ least 48 ours top bleed the system (while the pump is running of course).
http://www.pimprig.com has a great T line makup in their guides that i use myself. Never had a bleeding/filling/spillag problem with it. and it looks really clean, and since i have a mid-tower, it doesn;t eat up the space of a res.