growing 2 plants in the same bucket is not good practice unless they are clones, im not saying its impossible to do but it will be a ball ache especially if they both want different feeds which is probably why you have ph swings
when you make a res change and adjust the ph into range you should wait 1 hour and check the ph again to re-adjust, i use ph down that contains sulphuric phoshporus all the way through the grow with no ill effects i understand the nitric acid for veg but in veg you ph should be 5.6-5.7 then when they show sex move it to 5.8
you have the same background ec as me and by week 3 you should be on 1.6 ec, im pretty sure you are feeding your plants in equal amounts of micro/grow/bloom with 30ml of rhizotonic
double check the accuracy of your ph pen by making sure its configured properly
also another good indicator of ph and ec movement is the temprature of your water, if your using cold water on your res change as it warms up inside your grow room the ph an ec will alter, your res temps need to be between 65-75 degrees
these are just a few pointers for you to check out, if you do all those checks and the problem is still there get back to me
seymour