Below are some plans for a unique weapon in the
D&D 3.5 and Pathfinder game systems.
I need help to see if it makes sense.
The jist of the weapon is to have a flail
attached to a light crossbow in a way that you could
use it for a bashing weapon if out of ammo OR
detach the flail completely with such swiftness
that it will only take a partial action in a combat
round to detach and still be able to beat someone up.

The other thing is that this player wants to use
2 of them at a time... so how long should it take
to detach two flails from the light crossbows;
AND can you finesse the latching mechanisms in a way
that would allow you to detach completely with just
a trigger and no other hands needed?

so is this feasible?
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Plans for Revised Crossbow-Flail Combo Weapon:

Player wants to rework his signature weapon so
that the flail chain is kept completely out of
the way when he's not using it, but he has instant
access to it when he wants to use it (in game terms,
he wouldn't need to expend an action to ready the
flail).

The stock of the crossbow is slightly thicker than
normal and hollowed out. The flail chain is anchored
to the top of a spring-loaded piston. The piston's
spring is in turn anchored to the interior back end
of the stock. A locking mechanism holds the piston
in place with the spring compressed, and while the
lock is engaged, the flail chain is fully contained
within the stock. The flail's spiked ball sits at the
front exterior of the stock when the chain is retracted.

An external trigger releases the lock; when released,
the spring extends, and the piston pushes the flail
chain forward. At full extension, the flail chain
sticks out the from the front of the stock at the
same length that a normal flail's chain does from
its handle. A small, external turning wheel retracts
the piston, compressing the spring and drawing the
flail chain back inside the stock; once fully retracted,
the locking mechanism can be engaged. The mechanisms
for the crossbow are on the top side of the stock,
the trigger for the piston is on the bottom, and
the turning wheel on the side.

The firing of the crossbow is unaffected by the
flail chain being retracted or extended.