Peter:
You want to use the VA-1Axx unit from Electronics International. The xx indicates the size of the shunts needed, which depends on which alternators you have. You can see the unit at
http://www.buy-ei.com/VA-1A.htm . List price is $342 (USD). You also need a switch RSVA-3 so that you can monitor each engine on the single instrument. List price for the switch is $98 (USD). You also need two shunts. The S-50 shunts are rated for 50 Amps, and their price is $35 (USD) each.
Hope this helps.
Mark