They will start compete with each other for space, light and flow at some point and one of them will win this fight in the long run. This doesn't necessarly mean they will engage in direct chemical warfare but one may stop growing in that direction while the other changes direction upwards blocking light for the other. They may also directly fight with each other resulting in localised bleaching of one species.
Solution is also simple create a frag of both monti's from these facing sides before this happens and they have new growing space. Alternative is watch/enjoy the fight and see what the result is, it isn't always bad. I have a staghorn that is 'surrounded' by a Monti capricornis that came growing from below it and 'encapsulated' the single base stem of the Acropora (with an inch of space between them). It now seems to be growing out of the Monti giving a nice contrast in color between them as well. Just means I have to 'frag' certain parts of the Montipora from time to time to prevent it from 'encapsulating' more of the Acropora.
In this case I would not wait and see given the similar growth types and just create frags from these sides.