Adding a second country can be the best value decision of the whole trip, or the reason nobody enjoyed either one. The difference is the transfer between them.
Pairings that work
Baku with Tbilisi, an overland or short hop with two very different cities. Dubai with Baku, a smart stopover routing. Thailand with Malaysia and Singapore, three stamps on cheap regional flights. Turkiye with Azerbaijan, one visa strategy and a direct connection. Kuala Lumpur with Bali, a beach half after a city half.
Pairings that only look good on paper
Anything where the internal flight eats a full day, or where the second visa takes longer to secure than the first. Two long haul destinations in ten nights is a tour of airports, not a holiday.
Rules of thumb
Three nights minimum per city. Fly between rather than drive when the road is over five hours. Book the return from the second country rather than backtracking. Get both visas moving at the same time, not one after the other.