Tramadol mixes pretty well with most classical opiates/opioids.
I'd be wary of mixing Tramadol with things like Opana, Methadone, Demerol, Fentanyl, since most of those can cause seizures, as well (reference PDR 2016 ed.) on their own.
But Hydrocodone mixes well, Codeine, Morphine, Hydromorphone I believe all mix pretty well with tramadol. Though i'd probably rather mix them with Adderall, and just save the Trams for speedball withdrawal lol!
They definateley have good synergy, though too much APAP (like 6+ 325s at once) may awaken the demon that is "Tramadol Nausea" ahhhh....so spread em out, you'll be guch....gooch malooch.