yes you can, you need a phasing harness, If the Yagi's are normally fed with 50 Ohm coax then you make a phasing harness out of two electrical 1/4 wavelength ( at the freq of use) lengths of 75 Ohm coax. to a central T connector then 50 Ohm coax down to the transceiver.
ok the 2 x 75 Ohm lengths of coax act as impedance transformers increasing the impedance from 50 to 100 Ohms. So when these 2 75 Ohms ends are tee'ed together
(2 x 100 Ohms in parallel) we again get 50 Ohms to match to the feedline to the transceiver
ok for getting lengths will vary a bit on the type of 75 Ohm cable and the freq you are using ( you havent said)
wavelength λ(metres) = 300/ freq in MHz
eg. λ = 300 / 144 MHz = 2.083metres
divide by 4 = λ/4 wave = 2.083 / 4 = 0.52m
now we need the velocity factor of the 75 Ohm coax you will need the specs on the cable you use. it might be 66% (.66)
so 0.52 x 0.66 = 0.344metres
there's a start for you
ohhh .... say you have a 8 element yagi that will be ~ 10 dBi gain
phasing a second 8 ele yagi will give an additional 2.5 - 3 dB of gain, is approximately doubling the gain.
The last thing to comment on is spacing of the 2 yagis.
As a rule of thumb, ~ 1.25 - 1.5 wavelengths is the norm. This give the best tradeoff between greatest forward gain and smallest sidelobes
cheers
Dave