It’s pretty common to be able to use your own nameservers. The only registrar that doesn’t allow this afaik is cloudflare. I’m sure there’s probably others that don’t allow this, but most that I have seen seem to allow you to use your own nameservers.
Why do you say you can only have 2 nameservers? I’m sure not all registrars / TLDs will support it, but you can certainly have more than that. I’ve personally had 5 before, but I’m pretty sure you can have even more.
I believe Hurricane Electric allows you to do zone transfers to their nameservers, so I think in theory you can use their nameservers as additional backups. The SOA records will match too because of this, but even if you did something crazy like manage RRs on different nameserver providers without zone transfers I don’t think this would be a problem (well, aside from it getting out of sync unless you’re really careful). The SOA records are mostly used for zone transfers afaik and resolvers won’t really care about them, so even if they don’t match everything should work, no?