Several good options, all of which cost money:
1.
SpanningSync to sync both to the same set of Google Calendars. Has the advantage of giving you full web access to the calendars as well.
2.
SyncTogether, which will directly synchronize not only calendars but also address book, etc. But: no free trial, so you have to take the plunge before testing.
3.
iSynCal, which I've never used but seems nice.