Combining bottles with the same lot number does not even require mixing. Just funnel them into the big container, if you have some reason to. Personally, I'd figure just leaving them unopened in an ammo can will keep environmental influences to a minimum, so that's what I'd do, and open them as needed.
Combining different lots has two drawbacks: One is that they do need blending, and if they have different burn rates your result will be a burn rate in between the two. The other is the age difference in the lots means they will now both be as fresh and have storage longevity equal to the older of the two.
These days, canister grade powders for handloaders are kept as constant in burn rate as possible to keep load book recipes valid, but in the past they varied more. Bulk powders used by large scale ammunition makers can still vary a fair amount (because they use pressure guns rather than recipes to determine charge weight, so the cheaper bulk grade makes sense for them to use). I mention that because you would NOT want to mix, say, canister grade WC844 (H335) with surplus bulk grade WC844. The resulting burn rate is almost certain to be different, and the older powder will determine the life expectancy of the combined powders. A surplus powder often is made surplus because it's too old to put into new ammunition intended for long term stockpiling, or else it is collected from pulled down ammunition that was made surplus because it had already passed its stockpile expiration date.
The aging issue becomes more complicated with canister grade powder because it is intrinsically shorter lived than bulk powder. This is because the burn rate is controlled by blending a new bulk lot with older faster or slower bulk lots, as needed to raise or lower the rate, to arrive at canister grade powder. So canister grade powder is only as young as the oldest lot blended in to control its burn rate is. This process occasionally backfires on the manufacturer, leading to
recalling a powder that was breaking down too soon after leaving the shelf.
All of this begs the question, can you blend powders properly without risk or damage and know that it is adequately combined?