Try
&overlay DATINGLUX SPARKLES 1 opacity 1 in 0.5 AND overlay DATINGLUX SPARKLES 2 opacity 0 in 0.5 THEN overlay DATINGLUX SPARKLES 1 opacity 0 in 0.5 AND overlay DATINGLUX SPARKLES 2 opacity 1 in 0.5 loop INFINITE times
Try
&overlay DATINGLUX SPARKLES 1 opacity 1 in 0.5 AND overlay DATINGLUX SPARKLES 2 opacity 0 in 0.5 THEN overlay DATINGLUX SPARKLES 1 opacity 0 in 0.5 AND overlay DATINGLUX SPARKLES 2 opacity 1 in 0.5 loop INFINITE times