You cant.
I have a rather modest suggestion, if Toady would be willing to add it.
There should be two new body token: "[FIREGLAND:#tiles:ConeofFire
o:temp
o]" and "[SILKGLAND:#tiles:ConeofFire
o]", which would determine that the body part is capable of producing either webbing or fire. You would construct a body part like this:
[BODY:DRAGON_GIZZARD]
[BP:DGIZZARD:gizzard][CONTYPE:UPPERBODY][INTERNAL][SMALL][FIREGLAND:24:45:10800]
[BODY:GIANT_SPINNERET]
[BP:GSPINNERET:spinneret][CONTYPE:LOWERBODY][SMALL][SILKGLAND:10:20]
Then, using the [f]ire key, the option to spit fire/webbing using the marksman system is available. In the above examples, the adventure would shoot fire that would consume 24 tiles in a 45
o angle from the point of fire at a temperature equal to the homeothermic value of a magma man. The body part will then need a few steps to refill its sac.
However, you could also do something like this:
[BODY:2GIZZARD]
[BP:RGIZZARD:right gizzard][CONTYPE:UPPERBODY][INTERNAL][SMALL][FIREGLAND:24:45:10800]
[BP:LGIZZARD:left gizzard][CONTYPE:UPPERBODY][INTERNAL][SMALL][FIREGLAND:24:45:10800]
Then, the character would have the ability to shoot two jets of fire immediately, but would then need to wait for at least one of them to refill after the other has.
Just a thought.