Here's how I'll do it;
1) Initially convert to avi via AutoGK, target 700mbs.
2) After converting both discs to avi, convert back to DVD via convertxdvd.
Both can now be accomodated within a 4.5 single layer disc.
Now if you are technically adept,and have video editing experiences, you can try to utilize the
Hj-join software HJ-Join Description
HJ-Join is a freeware utility for software distributors
(and other people) who want to send and distribute large files.
With this program, others can automatically and easily
join the split parts of these large files back together
again. These split parts were split using HJSplit.
for tutorials kindly google the topic and you will be provided with a link to the site.