How much will it be to build two instances in Lightsail

Hello.

If you create two Lightsail instances on the $3.5 plan, it will cost you $7 per month.
The $3.5 fee is for one Lightsail instance, so if you launch multiple instances, you will be charged accordingly.
https://aws.amazon.com/lightsail/pricing/?nc1=h_ls

Hi

Lightsail plans are billed on an on-demand hourly rate, so you pay only for what you use. For every Lightsail plan you use, AWS charge you the fixed hourly price, up to the maximum monthly plan cost. The least expensive Lightsail plan starts at $0.0047 USD/hour ($3.50 USD/month). Lightsail plans that include a Windows Server license start at $0.01075 USD/hour ($8 USD/month).

So, for two instances cost is based on the how long you run them each month.

You can find the example calculation https://calculator.aws/#/estimate?id=0a2cb3c1a829304c9aa24ff908c65066cbe9c52b