The Constitution is pretty clear on the issue of the power of the Several States with regards to choosing electors:
The executive power shall be vested in a President of the United States of America. He shall hold his office during the term of four years, and, together with the Vice President, chosen for the same term, be elected, as follows:
Each state shall appoint, in such manner as the Legislature thereof may direct, a number of electors, equal to the whole number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or person holding an office of trust or profit under the United States, shall be appointed an elector.
But due the apparent nature of this bill, that it would not go into effect unless other states participate, would it violate Article I Section 10?
No state shall enter into any treaty, alliance, or confederation; grant letters of marque and reprisal; coin money; emit bills of credit; make anything but gold and silver coin a tender in payment of debts; pass any bill of attainder, ex post facto law, or law impairing the obligation of contracts, or grant any title of nobility.
Interesting legal issue. More than likely, the power of the Several States to determine how they choose their electors to the College wins out but I can see the argument where this is an unconstitutional alliance.