You can use /psort to sort your party list into a regular pattern. It always puts you in slot 1, but then the rest are configurable (like: tanks first, then dps, then healers). If you use either of the patterns that put tanks first, the tank will be in position 2 and you could use:

/psort
/assist <2>

The /psort line will make sure your party list is in the right order for your tank to be position 2, and then the /assist line will cause you to target whoever/whatever the player in position 2 is targeting. This relies on tanks being first in your (configurable) listing of which order you want players sorted into. A bigger issue is that if you're in a full party with two tanks, there's no automated way to control whether it's your main tank or off-tank who will have that slot 2. (The other will be in 3.)