If you're are talking about such as a Rickenbacker neck, they actually have both - 2 truss rods, both dual action - the rods are U-shaped & push as well as pull on the rear of the aluminium bar in the headstock.
pic: Haze Guitars
If the neck is level [ie not twisted], adjust both by the same number of flats, one at a time, left a flat, right a flat [if it even needs as much as a flat - I have been known to take 3 goes to pull a whole flat ].
If it isn't, but this would be pretty rare, then add more tension to the side leaning forwards the most.
Do it one flat at a time. Don't rush, don't pull it too far in one go.
I was reminded by the link to Haze that it's best to physically pull the neck back, then take up the slack in the truss rods.
Once you are getting close, then keep it strung to correct tension [with the strings you will be using] & let it rest a week or two before re-adjusting. It will settle over a period, so don't go mad with it the first time.
Rikki necks are quite springy; you can push/pull out of tune quite easily, but their overall tension remains constant for a long time.
One thing you might find with an old Rickki is that you run out of thread. If either nut goes tight as you're pulling the neck forwards, take it to a luthier. There is also the chance that some previous owner over-tightened it & broke one rod. If it's all already very tight & you need to come forwards some more - again, see a luthier.
Hmm… I just did a quick search - it seems the newer Rikkies have standard truss rods, though still as a pair - ref Haze Guitars - Rickenbacker Truss Rod Adjustment
