Every legitimate agent contract allows you to have a separate agent for a different genre, if the agent does not handle that genre. Not allowing it would be saying that you could never write in a genre the agent doesn't handle. Agents must allow this.
Though I'd never sign a contract with an agent. The ones I've had do not use contracts, and we're both better off for it.
I'm pretty sure my agent is legitimate, and my contract with the agency does not specify genre nor does it specifically give me permission to look for another agent for other genres. My agent doesn't handle romance, for example; the contract doesn't either. What it does do is make my agent and the agency my sole representatives for rights in all of my literary works.
If I decided I wanted to write a romance, my agent would handle it--he does know several romance editors, he just doesn't work with them in general--or he would talk to another agent he knows & trusts and ask them to do so or to work with him on it. I certainly wouldn't be in a position where I had to cold-query other agents.
I was perfectly happy to sign with him knowing he didn't rep romance, because I no longer write romance and don't see myself going back to it. But I don't think I would have signed with an agent who only represented one of the genres I wanted to write. Of course that's just me, and you obviously feel differently. I wonder, though, if you're querying exclusively agents who only handle SFF and not romance? Or are you thinking of querying agents who handle both, but specifically only for SFF?
As for "street cred," I wouldn't think so. It's going to come down to the work itself. What would probably give you a bit more "cred" would be if your romance novel has sold, and you could mention that. But I'd think more agents would be wary of the situation than would be impressed with it, because most agents want to handle a writer's entire career. That's just my feeling, though.