Bad idea.....depending on what you're doing with it. Without compatibility testing lubricating a bearing where the grease is supposed to last a long time or the application is severe is not a good idea.
In this case you have two greases that are general purpose greases. The Hi Temp is probably a Li complex with a dropping point over 260C/500F. Unless it contains some exotic base fluids it is likely to have a maximum service temperature of about 177C/350F. The other grease may or may not be a complex, you can't tell without testing (IR to confirm soap type and dropping point).
There are two considerations in grease compatibility, mechanical stability which is usually thickener related and chemical which relates to properties such as extreme pressure and corrosion resistance.
Testing is generally done in 100:0, 90:10, 50:50, 10:90 and 0:100 blend ratios. Mechanical stability is measured by the changes in penetration, 60 strokes versus 100,000 strokes. At times work can be applied by the roll stability apparatus but a standard grease worker is usually used. EP and corrosion resistance are then measured on these blends by tests such as the Four Ball EP Weld Load and the D4048 Copper Strip Corrosion test. In all cases you are looking for significant changes or loss of performance.
Two greases can be incompatible yet remain homogeneous when blended together. The separation tests are not good indicators.
In this case, the lithium thickeners will probably be compatible but you don't know about chemical compatibility or, for that matter, base fluid compatibility. I would not blend these greases. Use them separately or dispose of them.
