The simplest way I see for Splinterlands to reward both players is by treating both players of the match as the winner and then lowering the rewards by a set percentage for the loser or both.
For example, let's say there is a match between player A which would get 0.1 SPS and 121 GLINT if they won, and player B which would get 0.2 SPS and 245 GLINT if they won.
If only the defeated player would get their reward lowered by something like 90%, player A would get 0.01 SPS and 12.1 GLINT if they lost while player B's reward would stay the same.
The other option would have both players receiving a lowered amount of rewards, such as player A getting 0.09 SPS and 108.9 GLINT for a victory, and player B getting 0.02 SPS and 24.5 GLINT.
In both cases, the players would be rewarded for playing no matter how weak they are, but the stronger player would get a larger reward.