Love two different people at the same time

As Vignesh Natarajan already said, that feeling of caring for someone from the bottom of your heart is infinite. If anything, it only multiplies when you love more people, so that you are left with more than you started with. Love is not a scarce resource and it defies mathematics.

I disagree with the rest of Vignesh's answer. It is a huge drain on relationships if both partners hold themselves to the ideal of being everything to one another. Humans just don't work that way, otherwise nobody would need more than one friend. If you're lucky like me, you'll share an extraordinary amount of interests and activities with your partner. However, there will still be things only one of you likes, and insisting on sharing those as well will make one of you unhappy: person A for having to spend time on an activity they dislike or person B for not getting to spend enough time on activities they like. By contrast, if you abandon the idea of being everything to someone, the quality of your time together will go through the roof, because almost all the time spent together will be spent on things you both enjoy (except for unavoidable evils like visiting the in-laws). This way, your relationship becomes a collection of all the happiest times in your life.

The relationship will not become uncommitted, and you can still expect your partner to be there for you in unhappy times. In polyamorous relationships, there is the same expectation of faithfulness (faithful to the rules you made together, rather than sexually faithful to one person) and reliability. Part of being in a relationship means that you're ready to put your partner's wellbeing above your own. However, sacrifice is best left for when your partner actually needs it, when it actually makes a difference. Looking back on your relationship, it shouldn't look like an endless series of sacrifices, that would be doing it wrong, trying too hard to be perfect for the other, and in the process losing what really matters: affection for the other and looking forward to spending time together. Spending too much time with one person can actually be detrimental to that, "familiarity breeds contempt", you take each other for granted, take the time spent together for granted. Your mental baseline becomes skewed: all your partner's good traits become the new baseline, so that you don't notice them anymore and only notice their flaws.
That is why many people report that going polyamorous has allowed them to appreciate everyone more. The baseline re-adjusts itself, allowing you to see again all the ways that each of your partners is awesome, all the things you used to see when newly in love.

I used to think one could only love one person and that falling in love with another must be a sign of diminishing love for the first person, but I learned from personal experience that that's just not true. You can love several people, especially if they're different.