It is legal. However, some airlines have a formula that doesn't allow you to do that on applications. Like the grid you fill out for CAL. If you use both, the final numbers do not ADD UP RIGHT.
So you'd have to see which way the airline adds the times.
That's kinda like the PIC question. I am typed in the airbus, but I fly as an FO.
So do I log it as PIC, or SIC? I personnaly log it as SIC, 'cause that's what I am. I DID NOT sign for the AC, or the release, so I am not the PIC. I know others who log it as PIC to build up that time. But if you look at jetBlue, and I think MOST, they want the time you signed for the AC.