Hmmm. Even once they are bonded, taking them outdoors daily for their exercise is not ideal. Ideally, a rabbit should have constant access back to their "safe zone" (their cage/habitat) whenever they are exercising. By taking them outside, it deprives them of their retreat area, and also forces them to exercise at times they may want to lounge. Well, actually, there is no such thing as forcing a rabbit to exercise if he doesn't want to. So what it does is it provides only that limited time for potential exercise but deprives them of exercising when they happen to be in the mood (when they may be in the indoor hutch).
Nothing wrong with taking them outside on occasion for fun, but they ideally should have regular daily exercise in the same area their cage is.
They should be able to come and go out of their cage as desired during exercise time - ideally.
The other option would be for the cage itself to incorporate enough space for them to exercise. That would mean it should be around 30 sq feet to include the exercise area. [from House Rabbit Society: " at least 8 square feet of enclosure space combined with at least 24 square feet of exercise space, for 1-2 rabbits, in which the rabbit(s) can run and play at least 5 hours per day."]
If you have a very roomy indoor habitat that includes exercise space within it, then the outdoor playtime isn't so critical and you'll know they are getting plenty of exercise opportunity everyday and for whenever they desire.
Aside from the difficulty with having an outdoor exercise area that is separate from their living area, the greater difficulty will be keeping them bonded through the process (assuming they do bond as hoped) . Anytime a bonded pair of rabbits - especially a newly bonded pair - are moved from one location to another (from confined hutch to large outdoor space [or vice-versa] ) there is a potential for them to have disagreements over the new space and damage or break their bond. Just something to bear in mind while going through the bonding process.
There are a number of members here on RO who have both cats and rabbits and they get along just fine. This isn't always the case but was wondering if you've considered the possibility. It would certainly simplify things for you if they could get along.