We have two internet cafes in both Mikrotik. To each of them connected to two providers a 1-speed Internet 1.5 mb / s, 2-City Backbone without access to internet at a speed of 100 mb / s.

The question of how to merge the Internet channel between 1 and 2 cafes using urban Backbone. such as 1,5 mb / s 1,5 mb / s = 3 mb / s

Set up the usual PCC configuration as per the wiki example, and modify it it on each router to mark packets coming in through the city backbone as destined for the true WAN connection. The usual PCC restrictions apply, you can read about them in the countless PCC threads this forum has seen.