Absolutely nothing stops parties that want to use a different root to do so. Signed or unsigned. If DNSSEC is added to DNS, then of course the other root will also be signed with DNSSEC. We will still get multiple roots, a split DNS, with the only difference that they both are signed. This because the way DNSSEC is deployed today (and there is no change in sight in near future), the verification of the signatures are made in the same full service resolver that is "choosing" what root (zone) to use. This implies if you want to use a different root today, you change the IP address(es) for the root servers. When using DNSSEC you change the IP address(es) and the public key(s) for this other root. So DNSSEC doesn't change this at all.