Can we aggregate two columns of covariance matrix into one such that if initial matrix 3x3 then final aggregated matrix is 2x2?

Question

I have to aggregate two columns of cov into one

| 1.2 0.3 0.8 | | 0.3 0.9 0.1 | | 0.8 0.1 1.6 |

I would like 2x2 matrix by combining first two columns into one

I would like 2x2 matrix by combining first two columns into one