From the manual snippet xrunner posted:
If the same measurement function is used by both Main and Vice Display, then it will measure only one time and update the display.
So this seems a fairly useless feature.
Right, WTF is the point of looking at the same DCV presented two times on the same display? I might have to make a support call to get a person to 'splain it all in plain English.
I selected DCV for main and DCI for second, and the meter did display both - but how do you connect the leads to accomplish the measurement? The DCI area displayed 0.000 uA. OK. So what. If I connect the test probes to a PS, it shows the DCV OK, but the current display shows Zero and still, how would you get a current measurement anyway? There isn't a separate current input (except for the 10 A input).
I don't know if it's the same on the 3058 (too lazy to download the manual) but on my 3068, low current (200mA max) is measured between the two black terminals, the right one stays the common (negative) and the left one is positive.
So technically, it would be possible to measure both current and voltage at the same time.
I personally regret the 3068 doesn't have this feature, I would have swapped my extra useless digit for that...
Unfortunately, I discovered it after the trial period.
The same goes for the capability to measure both DC and AC components of a signal which I also miss.
The 3068 is a very accurate DMM but you rarely need such precision anyway and I think the 3058 is much better value for general use.
That said, it's not a big deal, as you generally have multiple generation of DMM in your lab anyway, so plenty of possibilities to measure several things at the same time, but I guess it's important to know if such bench DMM is your unique reliable instrument.