You cannot do this because you practically you want to "squeeze" the 660 cm3 flow in a 330 cm3 pipe. Simple. The Leo's pipes are engineered for 2in2 not ment for 2in1 system. You will effectively "suffocate" the bike.
If you want 2 in 1 you have to buy full system or find a pipe from 660 or higher capacity bike.
This is to explain it simply

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that is how i feel about my XTR
Mods so far:
Hyperpro front springs; Hyperpro rear shock spring; modified air box; LED tail light; LED indicators + new relay; YZ pegs; Lucas aluminum bar; brisk spark plug; snorkel removed; DNA stage 1; second hand DynoJet O2 mod; crash bars; front and rear steel braided brake lines; soft EBC brake pads; mitas 07 tyres, 2in1 Metal Mule exhaust, Blocked AIS, tall OEM screen, OTR bash plate