I have a Fastbreak, but not two linked. However, there are two linked at Cactus Jacks in OKC. IMO, linked play is fun as a novelty, but there appear to be a bug or at least an oversight in the linked mode. For example, I started a linked game but a few second in the other player got called away to play a tournament game. Somehow, at the end of the game the other game had ~78 points to my ~56 even though the flippers weren't touched for 98% of the game. It really doesn't seem like that should happen. I don't think it was a mechanical malfunction as the same game scored normally before and after in non-linked mode.

My son and I played them linked at Ann Arbor. Cool, but I wouldn't drive another 4 hrs to do it...And that's if its free.....

I'm a big fan of NBA Fastbreak, but the linked game is a novelty at best. Timed play and shared modes flatten out the skill discrepancy between two players. Might be fun for non-pinball folks to compete on, almost like pop-a-shot.

Definitely not worth the money and space commitment of a duplicate machine, unless you have a collection like the Ann Arbor Pinball Museum.

It is not cool...I mean it's kind of fun once but really not exciting. The ingenious part of Fast Break is it puts a time limit on the game for operators to make more money. So...yeah.

I've got the linked games & the vast majority of the time they get played in linked mode. I have 4 kids between 6-11 and they like to compete with each other and with my wife & me. If you're interested in playing pinball head-to-head, it's a fairly inexpensive way to do it & it's well-designed and a lot of fun. I disagree with the statement that it's only a novelty, and it does offer excellent replay value. So my suggestion would be to grab another one if you can find it and the price is right.

Instructions for the link

http://mirror2.ipdb.org/files/4023/Bally_1997_NBA_Fastbreak_Linking_Kit_Installation_Instructions_for_Linking_Kit_58030.pdf

For those of you who aren't familiar. I just ripped this from RGP but I believe it was in Clay's guide as well:
the NBA Fastbreak link option is done
through the A/V board's serial port. Installing a serial port on
WPC-95 games is easy, and you can save the money by doing it yourself
instead of buying the kit. The WPC-95 A/V board comes with those two
serial port chips missing by default, so these chips will need to be
purchased and insert them into positions U22 and U24 on the WPC-95 A/V
board. U22 is a MAX239 RS-232 driver chip, and U24 is a 16C450 UART.
Digi-Key (www.digikey.com) is currently selling the MAX239 for $7.55
and the 16C450 for $5.60 (using a buffered 16C550 as an equivalent
part).
The pinouts for the A/V board are on page 9 of the schematics, but
here's a summary:
J607-1 - Ground
J607-2 - TX output
J607-3 - RX input
J607-4 - CTS
J607-5 - RTS
J607-6 - DTR
J607-7 - DSR
J607-8 - Key (no connection)
J607-9 - RI
J607-10 - DCD
For basic RS-232 operation, all that is needed are the first three
signal lines, and you should be able to talk to the board. If not
familiar with RS-232 interfacing, obtain a copy of the WPC-95
Schematics, as these go a long way in helping understand how the
system works.
If the chips are installed properly the operating system should detect
the board automatically and start sending audits out the port. It may
need to enable printouts in the Adjustments menu, I don't remember if
that option trips automatically.
That's all there is to it. NBA Fastbreak also used this port in a null-
modem configuration for the head-to-head gameplay (swap TX and RX
lines between games). Linked game play works like this: the first
player presses Start, and their display shows "Waiting for 2nd
player." You can play a stand-alone game by pressing both flipper
buttons, or press Start on the second machine for a linked game.
Linked games are broken down into four quarters, with a halftime. The
quarter length can be modified in the menus. The gameplay is constant,
there is not limit to the number of balls (because it's a timed game).
If a player drains the ball, a new ball is served with no penalty
(other than the time this takes). The head-to-head players select
their teams and play begins. The players work together to complete
modes. For example, player 1 might complete the two left "in the
paint" shots, and player 2 may complete the two right "in the paint"
shots, which allows that mode's multiball to start. If both players
complete all modes and reach the final (wizard) mode, they compete for
the championship ring(s). If there is a tie the game goes into an
extra overtime period.