As always, John gives good advice. If the Seagull Spacewalker is the 120 size, then it will handle a little extra weight by using a second switch harness and two flight batteries. The advantages are a little redundancy for switch failures and slightly in a failed battery. I use two 1100 mah packs in such configurations.

As John points out, 5 cell packs would be better and is recommended for 2.4 systems. As well, the DS-821 servos will provide a little more torque overhead with 5 cells and if the Spacewalker is the 120 size, the 821s at 4.8v are on the lower edge of torque requirements. Another issue is cold weather and NiMH. They don't output near as well when cold, so if using NiMH the use of two flight packs halves the cold restrictions or constrictions.