The total cost of a planeswalker's loyalty ability is calculated before any counters are added or removed. If a loyalty ability normally costs [-3] to activate, you do not remove three counters from it and then put one counter on it. You remove two counters at one time when you pay the cost.
If you somehow manage to control two Carths (perhaps because of Spark Double), the cost-changing effect is cumulative. In total, loyalty abilities will cost an additional [+2] to activate.
If an effect replaces the number of counters that would be placed on a planeswalker, such as that of Vorinclex, Monstrous Raider, that replacement happens only once, at the time payment is made.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Donato Giancola
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
If a planeswalker's loyalty ability normally has a cost of [+1], Carth's ability makes it cost [+2] instead. A cost of [0] would become [+1], and a cost of [-6] would become [-5].