#These data are the same for CaseA and CaseB:
df$data1 <- log(explanatory variable)
df$data2 <- explanatory variable
df$data3 <- explanatory variable #(between 0 and 1)

#These data are related to the Case, IE CaseA or CaseB.
df$data4a <- measured value #(between 0 and 1)
df$data4b <- 1 - df$data4a  #(between 0 and 1)
df$data5a <- measured value #(between 0 and 1)
df$data5b <- 1 - df$data5a  #(between 0 and 1)


df$perc_a <- measured value #(between 0 and 1)
dfb$perc_b <- 1 - df$perc_a  #(between 0 and 1)

ModelA <- lm(perc_a~df$data4a+df$data5a+df$data1+df$data2+df$data3,data=df)
ModelB <- lm(perc_b~df$data4b+df$data5b+df$data1+df$data2+df$data3,data=df)

stepA <- step(ModelA)
stepB <- step(ModelB)

stepA$call$formula
RETURNS:
perc_a~df$data4a+df$data1+df$data3

stepB$call$formula
RETURNS:
perc_b~df$data4b+df$data5b`