There are plenty of TLH doubles with cross-bolts. One in my safe at the moment is William Powell's own patent I believe (at least the Powell ledger entry referred to it as "our crossbolt"). The bolt is square in cross-section, fitted low within the standing breech, and locks onto the top of an extended rear lump, rather than a rib extension. Its a plain gun, but made up in 'best quality' according to the ledger. Lovely stuff!

And BTW, I believe the Jones-patent rotary under-lever does not lock the barrel-flats down onto the action-flats, unless it is off the face (or about to come off the face). When new, the rotary under-lever locks the barrels down onto the standing breech-face, the same as in a top-lever gun. This misconception has arisen because a Jones under-lever gun can be off the face but still feel tight (ie not rattly) when it is 'down on the flats'. This is probably why it is regarded as a 'stronger' action, because far fewer under-lever guns are buggered enough to actually 'rattle', even though they may be technically 'loose'.