Sandbox

SELECT 
  a.variant_id, 
  a.option_id, 
  a.position, 
  a.modifier, 
  a.modifier_type, 
  a.weight_modifier, 
  a.weight_modifier_type, 
  b.variant_name, 
  IF(
    shared_option_variants.variant_id IS NOT NULL, 
    shared_option_variants.modifier, 
    a.modifier
  ) as modifier, 
  IF(
    shared_option_variants.variant_id IS NOT NULL, 
    shared_option_variants.modifier_type, 
    a.modifier_type
  ) as modifier_type 
FROM 
  cscart_product_option_variants as a 
  LEFT JOIN cscart_product_option_variants_descriptions as b ON a.variant_id = b.variant_id 
  AND b.lang_code = 'en' 
  LEFT JOIN cscart_ult_product_option_variants shared_option_variants ON shared_option_variants.variant_id = a.variant_id 
  AND shared_option_variants.company_id = 2 
WHERE 
  a.option_id IN (635, 631) 
  AND a.status = 'A' 
ORDER BY 
  a.position, 
  a.variant_id

Query time 0.00052

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	a	range	status,option_id,option_id_2	option_id	6		7	Using index condition; Using filesort
1	SIMPLE	b	eq_ref	PRIMARY	PRIMARY	9	mahm3t_cs443.a.variant_id,const	1	Using where
1	SIMPLE	shared_option_variants	eq_ref	PRIMARY,company_id	PRIMARY	7	mahm3t_cs443.a.variant_id,const	1

Result

variant_id	option_id	position	modifier_type	weight_modifier_type	variant_name
15239	631	0	A	A	240 mm
15240	631	0	A	A	300 mm
15259	635	50	A	A	50 mm
15260	635	75	A	A	75 mm
15261	635	100	A	A	100 mm
15262	635	115	A	A	115 mm
15263	635	150	A	A	150 mm
15264	635	175	A	A	175 mm
15265	635	200	A	A	200 mm
15266	635	240	A	A	240 mm
15267	635	300	A	A	300 mm
15268	635	365	A	A	365 mm
15269	635	400	A	A	400 mm