If you use 2 diffusers on 1 bottle, you will run out of the mixture just under twice as fast as if you used 1 diffuser on the bottle.
Remember that all diffusers are not made alike, so if you use a T joint, and it is easier for the gas to pass through 1 diffuser than the other, the second diffuser will work much less than the first.
I'm a bit weary of using plastic any thing with CO2 as CO2 destroys common plastics, like those used in T joints; after a while of exposure.
If you would like I'd be happy to send you some yeast as well ha ha.
A 3kg packet of sugar costs like 4 bucks? Maybe if you did the dishes or vacuum the carpets or something else like that you could trade that job for a 3kg packet of sugar that will be yours. if you are as good as bargaining as I was you'd get more than just 1 packet