CAR, NAR and TRA allow for the use of not more than 2 spacers with any reload kit.
The assembled motor weight of a longer motor casing and the weight of the spacers is greater than the optimal lenght casing, the thrust to weight ratio of a reload in a longer casing with a spacer or two is lower than the optimum length casing without a spacer. For example, a single space and a 1 g longer Pro38 casing weighs 50 grams more than the optimally designed casing. 2 spacers add an additional 50 g, for a total additional weight of 100 g.
https://pro38.com/pdfs/Pro38_Case_Spacer.pdf
This extra weight will lower the apogee compared with the optimally designed spacer and may require a 2 second shorter delay so you need to run a sim with a 100 g addition rocket weight for 2 spacers with a Pro38 reload with 2 spacers, but otherwise will not change the reload performance significantly.
Bob