If I wear my Instinct Solar (FW 17.50 and before) and my Swim 2 at the same time, the Swim 2 almost always shows a much lower stress level than the Instinct Solar.
The Swim 2 is not synced with GC. The phenomenon is independent of which wrist I wear which watch. Example values (Solar2; Instinct Solar):
35; 62
19; 51
52; 71
44; 45
22; 34
16; 37
37; 55
34; 65
The value for the body battery is therefore always quite low with the Instinct, often permanently at 5, or just slightly above.
A soft reset on the Instinct does not change anything.