I have to think in the viagra era this isn't all that difficult, but I don't think I've ever seen it done: a M/F scene with vaginal sex to creampie, and as soon as he's done cumming inside her, he pulls out and shoves it right back in her ass and just keeps fucking her (probably best to do all of this with her in doggy). Instead of the action stopping and the camera loitering in front of her pussy waiting for the cum to flow, you just keep filming the doggy anal action from underneath and eventually his load will drip out from him pounding her in the ass. He can either keep going until he runs out of steam or gets off a second cumshot to end the scene.