Paul,
I was thinking that the fact that the one key works for both the trunk and glovebox in our older, three-key-system cars pretty well demonstrates that the same style cylinder is used in both mechanisms. So yes, it seems a pretty good bet to re-key one to match the other.
And then, on the other hand, comes the Tech Manual entry that Iconic and 280SE Guy have referred you to, which points to other possible difficulties with that approach. Check it out before you drop serious money on locksmiths, get good counseling from the wisest-seeming of them and proceed with caution.
Maybe the best answer will be to do as you did with the glovebox: just get a key made for the trunk as well.
Let us know what you learn and how it works out.
Best luck,
Denny