No, the phone number and the contract are tied together, so you cannot do what you are describing. You'll either have to give up your current number when its shorter contract ends, or pay for the add-on line for 2 years if you want to avoid an Early Termination Fee.
Verizon does not allow you to switch numbers since they are tied to your contact. However, there's a work around, because Verizon does allow you to port a number from another carrier to overwrite the existing number without changing your contract. So here's what you need to do:
1. Get the Razr cheap like you wanted on a secondary line;
2. Wait for 4 months so your main line contract is up;
3. Port out your main line number to another carrier, for example, Pageplus;
4. Wait till your bill cycle is passed and port that number from Pageplus to Verizon, overwrite the number on secondary line.
After step 4 you will have your main number back on the line with the Razr.
How long would I have to wait between porting to pageplus and porting to razr?
Indirect retailers (such as Pageplus) have their own rules and regulations on plan changes, so it would be in your best interest to contact them to see how you can proceed with changing the numbers between 2 lines.
I'm not sure if it would break any contractual agreements with them.