It's fine to have both. The problem with caulerpa is that it does go sexual, as you stated. What this means is that during the process of nutrient uptake, the caulerpa grows and absorbs the phosphates and nitrates from the water. At a certain point in time, if you should neglect to trim it back regularly, the caulerpa can and will release the nutrients back into the tank (expel them). When it does this, it basically does it all at once and it's like dumping a super concentrated mixture of the nutrients into your system all at once. This process can crash your system in the span of just a couple of hours.
This is the reason that it's so very important to trim the caulerpa on a regular basis. Cheatomorpha will accomplish the same nutrient export as caulerpa, without the fear of it going sexual. That's why a huge number of reefers utilize cheato as opposed to caulerpa.
This is the very basic, layman's explanation. Hope it helps to explain your question.
swimmer