Air Carrier Captain reported a momentary loss of directional control during takeoff due to non standard procedures. The Captain reported handing control of the aircraft to the First Officer while the throttles were not evenly matched. When the auto throttles were engaged with unmatched N1's; the aircraft began to veer into the lower powered engine. The Captain re-synced the N1's and the takeoff continued.

Narrative

It was a dark; wet; early morning departure (XA15) out of ZZZ. A line of weather had just passed through the area and the rain had stopped 10 minutes prior to push back. ZZZ was departing on Runway XXL; with a slight tailwind of 2 kts (ZZZ winds 350/5 kts). We had computed our takeoff penalty at the gate for the tailwind and elected to perform a 26K max-power takeoff for the contaminated runway and slight tailwind.It was the First Officer's leg.... He is a relatively new hire. He requalified on the aircraft about six months ago.The taxi to Runway XXL was uneventful. Tower cleared us into position. I taxied onto the end of the runway; and pushed up the number 2 engine to assist with the 90 degree turn. Just as I was lining up with the centerline; and starting to retard the number 2 engine to idle; ZZZ Tower cleared us for takeoff. I gave the aircraft to the First Officer...saying; 'your throttle; your brakes; you are steering'. He pushed both throttles straight up. I saw the N1 'command sectors' indicators showing a trend to approximately 50% N1. The number two engine was spooling much faster than the number one engine since I had just had it spooled for my 90 degree turn onto the runway. I saw the split N1/Thrust scenario developing. I said 'Watch your Throttles' two times. At that very instant; the First Officer pressed the TOGA button; the number two engine rapidly spooled up toward the 26K takeoff thrust; while the left engine was lagging way behind. I repeated 'Watch your Throttles' and the aircraft started to head to the left side of the runway. I immediately took control of the aircraft. Disengaged the autothrottles; I pulled both throttles back to approximately 50-60% N1 and simultaneously straightened out the aircraft with the tiller wheel. I felt the nose wheel skip two or three times on the wet runway. While I was doing this the First Officer stepped abruptly on one or maybe both of the brakes; for a brief moment causing a slight nose down jolt.I now had the aircraft under control and pointed down the runway. I quickly scanned both EPR indications and saw that both engines were now spooled. I continued to push the throttles up to MAX power N1 'Chicken feet' then re-engaged the autothrottles. I handed the aircraft back to the First Officer; saying 'your aircraft'. The rest of the takeoff was uneventful. We discussed the importance of making sure both engines spooled; N1 gauges indicated approximately 40% N1 (bottom of the case) before pressing the TOGA button. I also reminded him that this was the exact situation that happened to [another aircraft] where they departed the runway. The First Officer wasn't really sure what had happened; as he was looking outside at the start of the takeoff roll and just saw the aircraft rapidly headed to the left side of the runway. Upon our arrival in ZZZ1; the First Officer performed a post flight inspection. No scuff marks were noted on the nose wheels.Weather; lack of daylight; and ops tempo.... We were late boarding due to the previous jet at our gate late push due to weather and reroute due to weather. I felt like things were moving quickly at the gate in order to get an on time 'out time'. I mentioned it earlier; communication; I probably should have been more precise in what I was trying to communicate to the First Officer.In retrospect there were a couple things I probably should have done to prevent this situation from developing. I take full responsibility for the situation I put my crew and the aircraft in. First; I shouldn't have handed the aircraft to the First Officer with the number two engine slightly spooled. I should have been more precise in my warning exactly pointing out that the number two is slightly spooled; instead of 'watch your throttles'. There was a slight language barrier as the First Officer's native language [was not English] (He does speak excellent English) and he might not have realized what I was hinting about with my 'Watch your Throttles' comment.... I should have been more direct.Finally; I should have kept the aircraft and completed the takeoff roll myself once the aircraft was heading back down the runway instead of passing control back to the First Officer as we were headed down the runway.