First, I don't know how 2 lenders can have a "shared 1st lien position". One of you has to be 1st and the other 2nd - unless this is a participation.
Second, the Flood rules indicate the 1st lien holder need only worry about their loan balance. Subordinate lien holders need to make sure there is enough insurance to cover them and lien holders in superior position.
If you're truly in 1st position, you could require $60,000 (the insurable value of the building) but the regulation only requires you to have $48,800 (your loan balance rounded up to the next $100).
P.S. You mention the RCV of $60,000 but you also say the NFIP max is $500,000. That tells me this is a non-residential, primary dwelling. If so, you shouldn't be using RCV to determine the insurable value. Only dwelling policies pay RCV. General policies pay ACV. If you'd like to know more about this, read the article called "Flood Insurance Insurable Value" in the "Free Downloads" / "Lending Tools" at our website:
https://www.bankerscompliance.com