Your concerns are a bit better separated if you keep your IdentityDbContext separate from your DomainDbContext, but then you would need to manage two sets of migrations (among other things). As you have it you'll only need one set of migrations. I wouldn't consider what you're doing "bad" necessarily- it really depends on the project.
If its any consolation, the project that we are currently working on uses only one DbContext that also inherits from IdentityDbContext as yours does. It does pull in some references to the Data Access project that I wish weren't there, but it does greatly simplify working with EF in terms of database generation, persistence, and migrations. Whether we'll outgrow it in the future or not is hard to say.