Possibly a similar situation to the Hensbarrow in Cornwall. The 2 hills in question are Mynydd Machen GW/SW-030 and the nearby HuMP Mynydd y Grug. At the moment Mynydd Machen GW/SW-030 is the official Marilyn with a height of 362m and Mynydd y Grug's official height is 355m, however the official summit of Mynydd y Grug is inaccessible as it covered with a spoil heap which increases the summit height of Mynydd y Grug to 375m which makes it higher than Mynydd Machen GW/SW-030. The OS Map even shows 360m and 370m contours lines of Mynydd y Grug. At the moment this Marilyn and HuMP won't be changing over yet as Alan Dawson says that Mynydd y Grug is more of a slag heap then a hill so fails the test to be a Marilyn. Here is his post below.
"One I am aware of is Mynydd Machen, which is probably lower than Mynydd
y Grug near Caerphilly, a natural hill with an artificial topping.
I have not been there personally but to me it looks more like a slag
heap than a hill and therefore it does not pass the test required to be
a Marilyn. It may mature, as some things and some thoughts do.
Further thought and inspection of Mynydd y Grug will be required in
future. Mynydd Machen GW/SW-030 is higher than the inaccessible natural summit (In
Pit or In Tip) but lower than the new summit, so this is one of many
grey areas, literally:
However there looks to be vegetation coming though the spoil heap so that is a good chance of Mynydd y Grug becoming a Marilyn in the future of Mynydd Machen GW/SW-030 getting demoted to a HuMP.