I tried both filters and filter groups, all translated into "AND".
Below, I intentionally used an invalid attribute to see the SQL generated.
1. Tried using 2 filters of same group
http://magento-dev:2080/rest/V1/orders?searchCriteria[filter_groups][customer_sales_order][filters][0][condition_type]=eq&searchCriteria[filter_groups][customer_sales_order][filters][0][field]=customer_id&searchCriteria[filter_groups][customer_sales_order][filters][0][value]=3&searchCriteria[filter_groups][customer_sales_order][filters][1][condition_type]=eq&searchCriteria[filter_groups][customer_sales_order][filters][1][field]=sku&searchCriteria[filter_groups][customer_sales_order][filters][1][value]=T760&searchCriteria[page_size]=10
Response body {"message":"SQLSTATE[42S22]: Column not found: 1054 Unknown column 'sku' in 'where clause', query was: SELECT `main_table`.* FROM `sales_order` AS `main_table` WHERE (`main_table`.`customer_id` = '3') AND (`sku` = 'T760') LIMIT 10"
2. tried using 2 filter_groups
http://magento-dev:2080/rest/V1/orders?searchCriteria[filter_groups][customer_sales_order][filters][0][condition_type]=eq&searchCriteria[filter_groups][customer_sales_order][filters][0][field]=customer_id&searchCriteria[filter_groups][customer_sales_order][filters][0][value]=4&searchCriteria[filter_groups][key_ring_sales][filters][0][condition_type]=eq&searchCriteria[filter_groups][key_ring_sales][filters][0][field]=sku&searchCriteria[filter_groups][key_ring_sales][filters][0][value]=T760
Response body {"message":"SQLSTATE[42S22]: Column not found: 1054 Unknown column 'sku' in 'where clause', query was: SELECT `main_table`.* FROM `sales_order` AS `main_table` WHERE (`main_table`.`customer_id` = '4') AND (`sku` = 'T760')"
