Hi,
I am afraid that's not currently possible, an arithmetic array can only evaluate the attributes of a generator to which it is attached. Depending on what you want to achiever there may be a hack. For example you can create a third generator that uses Line001 with a simple box in the Default input, wire this to a Transform node so that it doesn't actually create any geometry and you can then export the X,Y, or Z size and use that anywhere in the graph. This would give you spline length, but only if the spline was straight.
If you can provide more details of what you want to achieve, I might be able to help further.
Best,
Paul