What platform do you run Ardour on?
MDA has some Ringmodulator that works on all three (Windows, Linux, Mac), it’s free software and hence there are also distribution packages available. That has a fixed operand (sine wave LFO) though.
SWH has two plugin: “Ringmod with LFO” and “Ringmod with two inputs”. While those plugins run on all platforms, they’re mostly common on GNU/Linux (packaged as
swh-lv2 on most distros).
There are various other modulators, but most have a built-in generator. It’s not common to do “modular” style routing inside a DAW with separate generators.
Assuming you want to control both operand and carrier, a Bus would indeed be easiest.
swh’s “Ringmod with two inputs” has two inputs that are modulated (AM or RM) to a single output. On a stereo bus that will combine Left+Right inputs to mono output (which then can be panned).
So create a stereo bus, add the plugin to it, then connect the outputs of two Mono track to the bus’ L/R inputs (Using Window > Audio connections).
Another approach would to use the plugin directly on a Mono track, and then use a side-chain. “pin-management”  for each plugin allows to customize the signal flow and directly connect a signal from another tracks or generators to the plugin’s 2nd input.
 https://manual.ardour.org/signal-routing/signal-flow/ scroll down “Customizing the Signal Flow: The Pin Connection window”