How to draw a texture on another texture

this is a texture “TA” as a mask texture.and a texture "TB" saving alpha information. i want to add the alpha information from "TB" to "TA". i know shader can do this. but. once a new alpha information need to be added into "TA", I should writing a new Properties in shader _Mask0 ("Culling Mask", 2D) = "white" {}
_Mask1 ("Culling Mask", 2D) = "white" {}
_Mask2 ("Culling Mask", 2D) = "white" {}
_Mask3 ("Culling Mask", 2D) = "white" {} .... so i want to draw a new texture as mask texture that i only need to write one properties in shader.