You could probably use torch.chunk to get the paired kernels:
groups = 6
conv = nn.Conv2d(6, 12, 3, 1, 1, groups=groups, bias=False)
kernels = torch.stack(conv.weight.chunk(groups, 0))

# Reference
x = torch.randn(1, 6, 24, 24)
out = conv(x)

# Comparison
o1 = F.conv2d(x[:, :1], kernels[0], padding=1)
print(torch.allclose(o1, out[:, :2]))