I used the Transpac II for my intro to cave (w/single LP95) and loved it. When I used the same transpac for doubles (double LP95s), I didn't like it because it didn't seem to be that solid. Now I use a SS backplate and will never go back.
However, some cavers and wreck divers love the TPACII for doubles. I have also seen the Tpac harness modified for sidemount cave diving as well.
The most modular and upgradeable product out there for anyone wanting to ultimately move to more advanced diving is a backplate and harness. I agree with Wendy that you should get one wing for singles and another wing for doubles. I used the DR rec wing for singles and thought that it was overkill. For doubles IMO, the REC Wing is manageable but would rather use the Classic Wing for doubles. Over the past 4 years or so, there have been some changes in the technical gear market. Halcyon and now Oxycheq have great wings for single tanks, IMO much better than the current offerings of DR or OMS. I just purchased an OxyCheq for singles and will vouch for the product. Although you may wish to wait and compare the Halycon Pioneer and soon to be released Halcyon Eclipse to the OxyCheqs.
Since you are primarily interested in DR, why don't you speak directly with the DR rep in Canada. His name is John Reekie (spelling?). John is one of the most knowledgeable guys around and will give you a no bs response to honest questions.