Gauges generally ground through the senders. Look at the sender and count wires. If it only has one the sender itself is the ground.
Whether the sender is ground or wired to a ground I would check the fuse block first, it's probably a lot more accessible than the backs of the instruments and most likely feeds both. There are only a few fuses, pull each one and look at the contacts. If everything looks good there go to the gauges and work backwards. You'll probably have one wire on each gauge that is the same color, that should be power from the ignition switch, check the contacts. The other wire should be different color on each gauge and go to the senders.
If the sender has two or more wires it's pretty much the same to trouble shoot, fuse block, ign sw, gauge. The only difference will be the ground is discrete. You know what color wire came from the gauge, the other wire is ground. Check the contacts at the sender.
Depending on their proximity the two senders may or not share a common ground. If one connection has 2 wires and both senders have this color wire they're probably connected to the same ground point. Follow the harness, look for a wire that color to pop out and head off to a ground connection and check it.