A tunnel is a wide band, usually of neoprene, that is sewn inside a paddling top around the inside waist that seals the top of the pants (sometimes by rolling the fabric of the pants together with the tunnel but more usually just by acting like a large gasket to prevent water from creeping up into the top).
Since the tuilik is designed to keep you relatively dry and warm by being sealed around the cockpit coaming, most people I know who use one are not too concerned with sealing underneath it around the waist. When I had one (A Brooks) I just wore Hydroskin long sleeved tops beneath the tuilik and 1.5 mm Hydroskin long pants or Farmer Jane for my lower body. For moderate water temps I've worn lightweight Hydroskin (0.5 mm) tights under NRS Goretex paddle pants with semi-dry neoprene ankle and waist bands -- I got damp but not cold in that after swimming a couple of times.
Even if you wet exit with the tuilik, because the bottom is so open, it won't fill with water and bloat like a leaky or failed gasket drysuit would, so it isn't a hazard that way. And if yours is neoprene it offers insulation and added buoyancy anyway.
Greenland coach, Paolo Oullet, used to custom make tuiliks out of Goretex with a latex face seal, and he offered the option of including an inside tunnel which could be mated with the custom matching pants that he also designed and sewed. I had planned to order a set to replace my aging dry suit but by the time I was ready to afford it he had stopped producing them. I believe some of the two piece dry and semi-dry suits from Kokatat and NRS have that sort of interlocking tunnel option as well.