The first two rolls started to steer the game towards mutual priming. So, it's best to improve on both sides of the board by splitting the rear checkers.
  White is Player 2
score: 0 pip: 161  Unlimited Game Jacoby Beaver  pip: 160 score: 0
Blue is Player 1  
XGID=bECAAeCbdbB:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  24/22 10/9  eq: 0.093 
 Player: Opponent:  48.06% (G:12.99% B:0.64%) 51.94% (G:15.60% B:0.65%)  Conf.: ± 0.005 (0.098...0.089)  [100.0%] Duration: 9 hours 02 minutes 

2.  Rollout^{1}  9/7 8/7  eq: 0.116 (0.023) 
 Player: Opponent:  47.35% (G:14.05% B:0.69%) 52.65% (G:16.86% B:0.97%)  Conf.: ± 0.005 (0.121...0.112)  [0.0%] Duration: 8 hours 37 minutes 

3.  Rollout^{2}  13/11 10/9  eq: 0.172 (0.079) 
 Player: Opponent:  46.10% (G:13.84% B:0.73%) 53.90% (G:17.59% B:1.05%)  Conf.: ± 0.013 (0.185...0.159)  [0.0%] Duration: 1 hour 14 minutes 

4.  Rollout^{2}  13/10  eq: 0.185 (0.092) 
 Player: Opponent:  45.59% (G:14.06% B:0.74%) 54.41% (G:17.65% B:1.15%)  Conf.: ± 0.011 (0.197...0.174)  [0.0%] Duration: 1 hour 18 minutes 

5.  Rollout^{2}  10/9 6/4  eq: 0.187 (0.093) 
 Player: Opponent:  46.07% (G:12.62% B:0.67%) 53.93% (G:17.62% B:1.11%)  Conf.: ± 0.012 (0.198...0.175)  [0.0%] Duration: 1 hour 25 minutes 


^{1} 31104 Games rolled with Variance Reduction. Dice Seed: 10842456 Moves and cube decisions: 3ply Using version 1
^{2} 5184 Games rolled with Variance Reduction. Dice Seed: 10842456 Moves and cube decisions: 3ply Using version 1

eXtreme Gammon Version: 2.10
Splitting is even more urgent if white has a stronger priming structure arising after 43D31P21:
  White is Player 2
score: 0 pip: 163  Unlimited Game Jacoby Beaver  pip: 160 score: 0
Blue is Player 1  
XGID=bECAAeCbdbB:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  XG Roller++  24/22 10/9  eq: 0.144 
 Player: Opponent:  46.65% (G:12.55% B:0.64%) 53.35% (G:15.86% B:0.65%)  

2.  XG Roller++  9/7 8/7  eq: 0.199 (0.056) 
 Player: Opponent:  45.22% (G:12.93% B:0.58%) 54.78% (G:16.83% B:0.99%)  

eXtreme Gammon Version: 2.10
However, in 43D64P21 (the second roll is misplayed) white has an attacking structure, so splitting is not the best idea. (Rollout due to David Rockwell.)
  White is Player 2
score: 0 pip: 157  Unlimited Game Jacoby Beaver  pip: 160 score: 0
Blue is Player 1  
XGID=bECAAeCbdbB:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  9/7 8/7  eq: +0.094 
 Player: Opponent:  52.71% (G:16.03% B:0.76%) 47.29% (G:15.39% B:0.90%)  Conf.: ± 0.005 (+0.088...+0.099)  [100.0%] Duration: 2 hours 01 minute 

2.  Rollout^{1}  24/22 10/9  eq: +0.074 (0.019) 
 Player: Opponent:  52.31% (G:14.91% B:0.71%) 47.69% (G:14.89% B:0.50%)  Conf.: ± 0.006 (+0.068...+0.081)  [0.0%] Duration: 2 hours 05 minutes 


^{1} 10368 Games rolled with Variance Reduction. Dice Seed: 84824561 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10