My husband and I often travel separateley since he likes to ride his road bike. And we don't own cell phones at all!
We keep in touch by the crudest of methods: just agree on exact route and a periodic meetup point. If one or the other of us doesn't show up, t'other backtracks to find them. Some version of that may work for you if you don't want to keep to the cheapest option of just keeping each other in sight in areas of poor cell reception.
And there's always CB radios, which have become cheaper and cheaper! I'm sure quality varies, but you can buy them beginning at around forty bucks these days.
Here's one.
