Well I haven't found an answer to limiting it to 2 pages, but as (or more) importantly I found an article that ensures that a every new client starts on an odd numbered page - so that one clients record will never start on the duplexed side of another clients mailing. It took a while to find this so I'm sharing it here in case anyone else needs the info.
Start Record on Odd Pages for Duplex Printing in Crystal Reports :
There are times when you want all groups to start printing on an odd page number. Usually this comes up when you are printing on both the front and back of the paper. You always want a new group to start on the front side. The solution is to split the Group Footer into two sections (A and B). You put the normal "New Page After" on B. You don't check "New Page After" on A but instead you give the "New Page After" a formula condition:
Remainder (PageNumber,2) = 1
This way, when the group ends on an odd page, the report will put in an extra blank page so that the group ends on the even, and the new group starts on the odd.
In the attached document are more detailed instructions with screenshots.
