You can declare an array called textbox which holds 2 strings, then when a new item is added, you move the 2nd string into the first one, and you put the new one into the 2nd one.
Something along the lines of
dim textbox(2)
function addText(text$)
if textbox(0) = "" then textbox(0) = text$: exitfunction
if textbox(1) = "" then textbox(1) = text$: exitfunction
textbox(0) = textbox(1)
textbox(1) = text$
endfunction
Desktop: AMD Athlon XP2800+,Radeon 9800 128MB, 1GB DDR RAM
Laptop: AMD Athlon 64M 3000+,Mobility Radeon 9700 128MB, 512MB DDR RAM