Yes, we should let them develop before coming to any conclusions. I think a problem arises only if the Super Mario Wiki covers Donkey Kong so heavily that there is nothing DK Wiki can do to surpass them in coverage (for example, if Super Mario Wiki has covered every single thing involving Donkey Kong).
Will this ever happen? No one can really know for sure. But suppose that it does happen. By then, DK Wiki will be very well-developed and a lot of hard work will have been put into it. However, Super Mario Wiki will have the exact same thing. No rational person would go to a DK Wiki when there is a Mario wiki with the same quality, the same detail, but greater coverage.
This is a long-term problem (Super Mario Wiki is not even close to covering all things Donkey Kong). However, it's probably in the best interest of DK Wiki editors for a decision to be made regarding coverage right now. We wouldn't want all their hard work to be in vain two or three years from now.
Simply, the coverage issue will grow as the wikis do. In my opinion, to delay a decision would be unwise. It's best to tackle the issue before it gets too big. There are a couple options out there - a merge as equals, an absorption of DK Wiki, an end to DK coverage on SMW, etc.
This is a decision that the staff of both wikis will have to discuss. In the end, it is up to the wikis what they do, but it would be of great benefit to at least consider our opinions.