I might be of some help here.
I had one Titan (earlier version than yours but similar with slightly less output at lowest octave according to reviews).
Was happy. Very nice sub.
I then picked up a second Titan of same vintage (97-01).
Liked this very much and setup was easier in some ways as dual subs do a lot of positive things in a room.
Then ran across the dual LFM SE's which were essentially originally made to match my ACI Jaguar monitors.
These each have two 10" drivers (same as Force drivers) and are equal to 4 Forces or allegedly a couple of typical 15" subs.
At first I was a little unsure about the switch. They were faster as you note.
They were not as weighty as two Titans at first.
After about a week, they broke in one night and they are now excellent in every way -weight/extension/speed/transition with the monitors/soundstage.
If you are looking for more bass weight in your room, depending upon room size then two Forces will give you more of that. These 10" drivers
are quick and two of them will give you more output than one 12" Titan, but if the room is large it might not seem like a real lot.
Now if your room is moderate sized (say 12'X18) then you will get a lot more impact
and speed and I think be very pleased with two Forces.
If you have a larger room (say 15'X30 with high ceilings) then the impact and weight would be an improvement, but depending upon your tastes may not be enough.
If your goal is speed and even response-then two Forces will be great in almost any room is my guess.
What am I saying?
Get four Forces.

Just kidding.
I think there is a pro review somewhere and the guy got two Forces if I recall and loved it.
I have a pretty large room (15X25 and it goes to 18' wide in back part) so I love the four 10" drivers.
You might also consider one Maestro or a second Titan also. I am guessing your newer vintage Titan is pretty quick as it is and the lowpass cutoffs allow nice setup.
I will say that dual subs is a great way to go in my opinion, but I love bass and ACI does it right.
Here is a thread on my setup with pictures.
http://www.audiocircle.com/index.php?topic=44090.0Good Luck.