Does SQL Server 2017 Express install on Dual CPU server?
Will it install?

Yes.

Will it work?

Yes.

Will it use all of the horsepower the server has available?

No. The limitation of the Express Edition -- "single physical CPU or four cores" -- means that SQL will detect more than one CPU and will set the others to offline. You'll see that in the results of select * from sys.dm_os_schedulers both as "VISIBLE OFFLINE" in the status column and as "0" in the is_online column.

So you and your customer can do this, but you'll be wasting some money. The recommended approach all depends on what the price difference is between the dual-CPU system and a similar single-CPU system. If it is a small cost difference, and there is a strong possibility that they will upgrade to Standard Edition or Enterprise Edition soon (after all, you do mention that Express Edition is enough "for the time being"), you might want to advise them to start with the dual-CPU system to enable making that future edition upgrade without also having to deal with a server upgrade. If the price difference is not insignificant, go with the single-CPU system so you don't waste the cost of the second CPU.