A recent urban report suggests that carrying two handsets could be regarded as a security threat.
Doubtless this is an exaggeration as many people use two phones to split calls between office and home, especially as using the work mobile for personal calls is taxable.
Orange had an answer to this years ago with two numbers active on one SIM.
You could of course use a dual SIM holder, but only one line would function.
Perhaps a perfect solution is from WND Telecom whose handsets allow two active SIM’s in one phone.
Though this may confuse the authorities!