Picking your two instruments
The most common mistake is picking two that move together, which gives you one position twice.
Whatever the constraint — a subscription that covers two, or simply a limit on what you can follow properly — choosing which instruments to work on is a decision worth making deliberately rather than defaulting to whatever is most talked about.
Start with what you already watch
Familiarity is a real advantage that is hard to acquire quickly. Knowing what an instrument does at the open, how it behaves around releases, and what an ordinary day looks like is context no tool supplies. If you have been watching something for a year, that is an argument for it.
Liquidity is a threshold, not a preference
An instrument that does not trade enough is not a candidate at any price. Everything in liquidity and spread applies. Rule out the thin ones first, then choose among what is left.
Match the hours you actually have
An instrument whose active period is when you are unavailable will produce missed setups and late decisions. This sounds obvious and is routinely ignored by people who pick an instrument because it is popular rather than because they can watch it.
Check the money per tick against your account
Tick value determines what a normal day's movement means for your balance. An instrument whose ordinary range represents an uncomfortable fraction of your account will produce decisions driven by discomfort rather than by the chart. Many contracts have smaller versions for exactly this reason. See tick size and rollover and position sizing.
The mistake worth avoiding
Do not pick two instruments that move together. Two index futures, or two large banks, will spend most of their time doing the same thing. You have not doubled your opportunities — you have arranged to take the same view twice, at twice the size, which is the correlated indicators problem in a different form.
The check takes five minutes. Pull up both charts over the last few months and look for periods where one went up while the other went down. If you struggle to find any, pick something else.
A reasonable pair
One instrument you know well and can watch during its most active hours, and one that behaves differently — a different asset class, a different session, or a different typical volatility. That gives you something to work on when the first is quiet, and it means a single market-wide move does not hit both positions the same way.
One more consideration: whatever you choose, expect to spend a few weeks learning what ordinary looks like on it before any measurement means much. That period is unavoidable and it is a cost of switching.