Which of the following letters will sequentially replace the question marks ($?$) in the given series?
G, Z, I, X, K, V, ?, ?

The given series is an alternating series consisting of two intertwined independent logic sequences:
1. First, third, fifth, and seventh terms (Odd positions):
$\text{G (7)}\stackrel{+2}{\to }\text{I (9)}\stackrel{+2}{\to }\text{K (11)}\stackrel{+2}{\to }\text{M (13)}$
2. Second, fourth, sixth, and eighth terms (Even positions):
$\text{Z (26)}\stackrel{-2}{\to }\text{X (24)}\stackrel{-2}{\to }\text{V (22)}\stackrel{-2}{\to }\text{T (20)}$

Combining these results, the next two missing letters replacing the question marks are M, T.