it seems pretty straightforward to me;
Yes it is, just be mindful of the consequences if for whatever reason you open one relay before closing the another.
For example how will you know that relay 1 has actually closed before you open relay 2?
What if there's a mechanical failure in relay 1 and it sticks open, if you then open relay 2 it's goodnight nurse to half your circuit and with the resultant fire maybe the vehicle as well.
Do you trust your coding ability to that extent?
If you use a 4514 decoder chip it's not possible to have > 1 output active at a time which at least takes your code out of the equation, not mechanical failure though.
Rob