The lower voltage charger does not add its full amps right up till when it is passed by the higher voltage charger. The lower voltage charger starts to taper in amps before the battery reaches the voltage of the other one. So they do add their amps right to the end but the total amps tapers until only the full amps of the higher voltage charger are still going in. Then those amps taper after a while too as is normal.
Actually you get all sorts of tapering. You can have both tapering but the lower one starts first. It depends on the proportions of their "spreads" between charger and battery voltage, battery AH, total amps, and SOC.