php – Use old legacy application code from new application

If you have a big legacy application (big ball of mud), lets call it MudApp.

If you then create a new application to make us of new technology and to make a new good looking frontend because that was impossible in the MudApp. This new application is supposed to the fresh one and with only the most important features of the MudApp. Lets call it MobileApp.

Some features is really difficult to implement in the MobileApp with identical functionality as in the MudApp so the solution has been to just make a request (via API, SOAP or whatever) from MobileApp to MudApp and let the MudApp do all the work. Some other features is written directly inside the MobileApp.

My first thought is that this is bad with to much coupling between them etc. But I can not really make an argument for why it’s bad.

Is this bad or good? And why?

Note 1: the applications is sharing database (which for some is bad practice, but in this case I think it’s fine because it would be to much work syncing to another database and so on.)

Note 2: The reason for the question is because the MobileApp is supposed to be further developed. Some features will be added and some removed. So I’m trying to figure out if the NEW MobileApp should make us of the current MobileApp (Took approx. 2-4 month to write) or just start over on a clean sheet