NN- same Number series for two different document types

Hi,

We can keep the same series for two different document types at the start. However, once we start generating the documents the sequence number changes as one document is used more than the other.  Eg. RI and RM, both have the same NN series (Co./FY) but since invoices are generated more than Credit orders the series become different. 

There is a requirement to keep them the same but document type still different.  Eg. if RI si 180000169, RM generated after that should be 180000170.

Is there some setup required to achieve this

Thanks,

Sameer