I have noticed that GFCIs seem to be worn out by heavy, prolonged loads. I can't find any authoritative sources for this effect. Probably the GFCI manufacturers know all about it.
The bridge circuit which detects the flow imbalance depends on some precision electronics which are heat sensitive. The heavy current itself does not affect them, but the side effect of that current heating up the components will lead to premature failure.
Normally, you can never split a 40A supply to serve two loads rated for a 20A circuit. Except here.
This is an allowed exception for supplying oven/range loads. It's in NEC 220.55, referring to Table 220.55, Note 4:
The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.
NEC 220.55 and Table 220.55 also includes some derating (or more accurately, permissive up-rating) that overrides the normal "125% for continuous" derate found in NEC 210.19(A)(1). It appears to be permissive. Speaking of 210.19, ThreePhaseEel points out
210.19(A)(3) Household Ranges and Cooking Appliances. Branch- circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8 3/4 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.
Exception No. 1: Conductors tapped from a 50-ampere branch circuit
supplying electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than
20 amperes and shall be sufficient for the load to be served. These tap
conductors include any conductors that are a part of the leads supplied
with the appliance that are smaller than the branch-circuit conductors.
The taps shall not be longer than necessary for servicing the appliance.
Exception No. 2: The neutral conductor of a 3-wire branch circuit
supplying a household electric range, a wall-mounted oven, or a
counter-mounted cooking unit shall be permitted to be smaller than the
ungrounded conductors where the maximum demand of a range of
8¾-kW or more rating has been calculated according to Column C of
Table 220.55, but such conductor shall have an ampacity of not less
than 70 percent of the branch-circuit rating and shall not be smaller
than 10 AWG.
This also overrides the 125% rating by saying a 40A breaker can definitely supply two 20A ovens, and saying a 8.75KW-9.6KW oven is allowed on a 40A circuit.
Best Answer
Depending a great deal on EXACTLY what you have now and how it's wired:
Possibly you could replace your current 60A outlet with a sub-panel and put two 30A breakers in the sub-panel for your two 30A outlets. That will only be the case if what you have now has 4 separate wires feeding it (Hot, Hot, Neutral, Ground - also known as L1, L2, N, G) or 3 wires plus a continuous metallic conduit which acts as ground.
If this is an old installation without a separate grounding conductor, you can replace the cable you have now with a cable with 4 wires, 4 wires in plastic conduit, or 3 wires in metal conduit (the metal conduit acts as ground) and still place a sub-panel. In that case you might want to consider using a larger size of wire and a larger breaker to feed it, to allow for future expansion, if your service / main box will support that.