Just a quick question for everyone: When you are building several high tier items, do you finish one and move on, or build all the medium tier items first?

For example, I want to build a Frozen Heart and a Frozen Mallet. Do I complete FH and then start building FM, or do I build Phage, then Glacial Shroud, then complete both?

it really depends on not just the items, but the characters. Zeals are nice, but two zeals are more expensive and provide less move speed bonus than a single phantom dancer- however in other ways the two zeals might be better, it depends on the needs.

In your case- what char are you using and what playstyle? you may find the completed frozen heart/mallet more useful than the phage/glacial combo in some cases and not in others.

generally I start with prospector item + boots + pots
then i build in this order

1)finish boots (usually merc treads, ninja tabi or lucidity)
2)get a defensive item based on who is doing the most damage on enemy team (chain vest or negatron cloak)
3) work on the core items for your respective champion
4) situational items to counter enemy build

now in your example you want FH and frozen mallet.
Generally I would choose to build phage first due to health + chance of slowing enemies + some dmg
after phage is complete move onto glacial shroud for CDR, mana and armor

after you have the 2 pieces it really is situational.
If the enemy team is heavy AD and you find your dying too fast i would finish the FH first as it gives more armor, CDR, and an aura that slows atk speed of enemies to be optimal.

If however your team is a bit ahead and you are playing a tanky dps, i would finish frozen mallet so once you initiate they can`t escape (assuming your team helps kill them of course)

This is actually false. The second zeal is 455 cheaper than the PD upgrade and gives 1% extra MS, which is why people stack zeals in the first place. The PD gives more offensive stats.

It depends on whether you're building for stats or effects. Glacial gives most of the cdr of FH, you finish it when you want the armour and aura. Giant's belt gives most of the HP of mallet and Phage gives most of the AD, don't combine it until you want the effect.