Legacy Diesels with the Same ID#

As Marty has stated above, you can assign any ID that you wish. (It is recommended not to use '1' or '99' though.)

I normally use the last 2 digits of the road number, and on the occasion where I have 2 with the same number, then one of them is changed to use the another number.  In your case, if you used the last 2 digits here, you would have 51 and 70. If you wished to continue to use the first 2 digits where possible, you could have 18 and 70, or 18 and 51.

As for the blinking lights, could you provide a bit more detail?

If these are recent Legacy units, then the number of 'blinks' normally indicates a diagnostic code. 1 blink means a stalled motor (Unlikely in this case, as you would probably have mentioned if the locomotive would not run.), 2 blinks is a smoke element problem (Also possibly a loose connection or a soaked smoke unit pcb.) and 3 blinks indicates a smoke fan problem. 

As Marty mentioned, a reset may cure this if there is not a fault.

I'm from the last 2 digit school. I've even gone so far as to use colored electrical tape with dry transfers to change numbers. This is a nice method because it is totally reversible.

Eventually, I want to do some repaint patches with water decals on some engines.

There is also the ugly but faster stick on numbers on the cab roof. I consider that to be a temporary solution, however, it is most visible when trying to see loco numbers in the hidden yard. I may end up doing this to all my engines, so they can be seen more easily on the video monitor.

I also use the last two digits for the engine number.  A friend and I both have Lionel Legacy Union Pacific heritage WP diesels that we operate on the same layout.  The engine number is 1983.   He uses 83 for his number and I changed my engines ID to 84 to avoid a conflict.

Joe

I use the last numbers and reverse them, less likely to run into a problem with you use the last two number guys

3985 becomes 58

1982 becomes 28

But it really doesn't care other than don't use 1 or 99

I use one or two numbers, in the order they appear on the cab. Since there can be a conflict with another engine, this gives me more choices and always results in success even if I have to try more than once. The choices are generally limited to 7 possibilities.