Arguably, these archetypes cannot be taken together
Although the guide and urban ranger archetypes don't, at first glance, alter or replace any of the same things, these run into an obscure rule about pathfinder archetype stacking that is found here and clarified here.
The long and short of the precise rules here is that because the guide archetype's Terrain Bond ability circuitously alters how Favored Terrain works (by granting the bonus to allies when in your Favored Terrain), it arguably counts as altering it. Thus, it's not compatible with urban ranger, since urban ranger replaces Favored Terrain entirely.
Even if the archetypes stack, the result is a rules dysfunction
If allowed to stack anyway, and run as-written, the following things happen:
- The guide archetype trades away Hunter's Bond for Terrain Bond, which only activates while the ranger is in his Favored Terrain.
- The urban ranger archetype trades away Favored Terrain for Favored Community, a different ability that cannot trigger the above bonus.
Thus, when you stack the archetypes, the Terrain Bond ability outright does nothing; you are never in your Favored Terrain (as you have none), so the bonus is never granted.
What I recommend
Overall, the archetypes don't really seem to have a balance issue if you let them stack and adjust some things. I recommend asking your GM to implement the following houserules:
- Guide and urban ranger can be stacked
- Terrain Bond applies to the urban ranger's Favored Community, instead of the nonexistent Favored Terrain ability.
Favored Community is functionally identical in effect—albeit far smaller in scope—to Favored Terrain. The only difference is the location you get buffed in, and that Favored Community adds a bonus to Knowledge (local) instead of Knowledge (geography). It's likely to be active about as often as Favored Terrain will (if you chose the terrains well), so it stands to reason that things shouldn't break if you allow the urban guide ranger to guide their allies through a city/ship/town/etc.