The Infernixsaid: You should be able to log in to OSRS and RS3 at the same time with the same account.
Unfortunately since OSRS and RS3 are completely separate games that use the same account, this is not something that can be done at present - as Tuffty has pointed out - and for the time being, it does not appear that Jagex has any plans to change this in the future.
- Last edited on 19-Mar-2019 22:09:05 by CM Nick
The account identifier is stored as one single entity, that I think is why your request is not technically feasible. It is like making yourself to be both in Paris and London at the same time. Not technically possible with the current setup of RS3 and OSRS.